The comments come at a delicate time for the Fed. Later this month, the central bank will decide whether the U.S. economy is healthy enough to handle the second interest rate hike since the Great Recession.

Trump is hardly the only person to criticize the Fed for keeping rates low for longer than is needed. Critics argue artificially-low rates will create another asset bubble, like the housing bubble that ended up crashing the economy last decade.

Rates were slashed to near-zero during the recession.

Others believe the low rates are necessary because the American economy continues to perform below its potential and is being hurt by turbulence overseas.

Trump has been somewhat inconsistent about his own stance on low rates.

In October 2015, Trump told the Hill that the Fed is forcing investors into an "inflated stock market" that is in the midst of a "bubble."

However, Trump also told Fortune in April that it would be "scary" if the Fed raised rates now. "The best thing we have going for us is that interest rates are so low," he said.

In any case, with a new CNN poll showing that Trump has erased his deficit against Hillary Clinton, the public criticism of the Fed may not sit well with the markets.

"The biggest wild card may be the possibility of a Trump dispute with the Federal Reserve, which markets would not enjoy," Greg Valliere, chief global strategist at Horizon Investments, wrote in a note. "If he wins, the prospect of a public spat between Trump and Janet Yellen will loom by spring."